In a bowl, combine the cornstarch, brown sugar, ginger and garlic powder. Stir in broth until smooth. Add soy sauce and molasses; set aside., In a nonstick skillet or wok, stir-fry steak in oil for 4-5 minutes; remove and keep warm. Stir-fry peppers, celery and onions until crisp-tender, about 5 minutes. Stir broth mixture and add to the vegetables. Return meat to the pan. Bring to a boil; cook and stir until thickened, about 2 minutes. Stir in lemon juice. Serve over noodles if desired.